In my co-pending application Ser. No. 603,357, filed Aug. 11, 1975, now U.S. Pat. No. 3,999,743, and Ser. No. 729,705, filed Oct. 5, 1976, now U.S. Pat. No. 4,096,588, there are disclosed methods and apparatus for recycling used asphalt-aggregate compositions. More specifically, in those applications there are disclosed methods and apparatus for directly exposing used asphalt-aggregate compositions to flame and/or hot gases of combustion. The apparatus disclosed, and which is of the type used in this invention, comprises an elongated rotating drum including lifters for alternately elevating the particles and then allowing them to drop in a cascading manner through the hot gases. Although the purpose of the inventions disclosed in those applications is to prevent overheating the asphalt-aggregate particles by separating the material to be recycled into different particle size ranges, and introducing the various particle sizes into areas of a heating apparatus where the hot gases of combustion to which the particles are exposed will be low enough to prevent overheating and degradation of the particles, it has been found that some volatilization of hydrocarbons results from exposure of particles to the hot gases in the processing. For example, where coarse composition particles are introduced into an input drum end where temperatures are at their hottest because of flame and hot gases of combustion being introduced into the drum at that end, for example, 1,000.degree.-2,000.degree. F. or more, some volatilization of hydrocarbons from the coarse asphalt containing particles will occur as they pass through these hot gases. In addition, it is also found that some smaller or fine asphalt containing particles will be present with the coarse composition introduced into the hottest input drum end, which fine particles have not become separated in the particle size separation processing, or have resulted from handling of the coarse particles as they are transferred from a stockpile for introduction into the heating apparatus. Since these small particles will become readily heated when exposed to the hot gases of combustion, volatilization of hydrocarbons therefrom during the direct exposure recycle processing may be unavoidable. This results in a visible smoke as the hydrocarbons condense in the cooled gaseous exhaust. It is also found that substantial amounts of inorganic or mineral particulate matter is present in the gaseous exhaust mixture. This particulate matter, mostly metal silicates, results from the aggregate present in the feed composition and is present in the exhaust in particle sizes of nominally 20 mesh and smaller. Accordingly, the resulting gaseous mixture created in such a recycle processing includes not only the relatively clean exhaust gases from the burner which has supplied the hot gases of combustion to the asphalt-aggregate heating apparatus, which gases include nitrogen, carbon dioxide, and water vapor, but also significant amounts of the asphalt hydrocarbon volatiles and visible inorganic or mineral particulate matter which should not be exhausted into the atmosphere where atmospheric pollution controls are required.
The treatment of such an exhaust mixture as described to meet pollution control standards is especially problematic. Because the gas contains little oxygen, but substantial amounts of carbon dioxide and water vapor as products of the burner hydrocarbon combustion, together with usual amounts of atmospheric nitrogen, it will not itself support combustion, even with the presence of the volatile hydrocarbons. Moreover, attempts to treat the mixture to remove the silicate mineral particulate matter, produced from aggregate fines in the feed composition, by conventional methods, is unacceptable. For example, wet scrubbers introduce even more water vapor into the gaseous mixture. On the other hand, a dry scrubber will not separate out the very small particles of below about 20-40 microns. Moreover, attempts to filter the gaseous mixture through a bag house quickly results in the hydrocarbon condensate plugging the fabric filter bag. Thus, it is to the treatment of this exhaust that the present invention is directed.